The magnetic resonance imaging (MRI) apparatus is a medical image diagnosis apparatus configured to apply high frequency magnetic field and gradient magnetic field to a subject placed in the static magnetic field so that the signal generated from the subject is measured and imaged through nuclear magnetic resonance. The MRI apparatus mainly has two types, that is, tunnel type of a horizontal magnetic field, and open type of a vertical magnetic field. The MRI apparatus of former type applies the static magnetic field in the direction parallel to the subject's body axis, and MRI apparatus of latter type applies the static magnetic field in the direction vertical to the subject's body axis.
The MRI apparatus is configured to allow acquisition of the image in an arbitrary imaging cross-section. The imaging cross-section includes three cross-sections orthogonal to one another, that is, a transverse section for separating the body into a head side and a foot side, a coronary section for separating the body into a ventral side and a dorsal side, and a sagittal section for separating the body into a left side and a right side as well as an oblique section for obliquely separating the body at an arbitrary angle.
Generally, the MRI apparatus is configured to apply a slice gradient magnetic field for specifying the imaging cross-section, and simultaneously to give an excitation pulse (high frequency magnetic field pulse) for exciting the in-plane magnetization so as to obtain the nuclear magnetic resonance signal (echo) generated at the stage where the excited magnetization converges. At this time, the apparatus further applies the phase encoding gradient magnetic field and the lead-out gradient magnetic field in the direction vertical to the slice gradient magnetic field with respect to the imaging cross-section within the period from the excitation to acquisition of the echo for the purpose of imparting the three-dimensional positional information to magnetization. The measured echo is placed in the space k with axes of kx, ky and kz, which is subjected to the image reconstruction through inverse Fourier transformation.
Each pixel value of the reconstructed image becomes a complex number including an absolute value and a deflection angle (phase). The absolute value and the phase are determined by imaging parameters including the static magnetic field intensity, the static magnetic field direction, type of the imaging sequence, the voxel size and the repetition time, the density of magnetization in the subject and relaxation times (T1, T2).
Normal diagnosis uses the grayscale image (magnitude image), taking the absolute value as the pixel value. The magnitude image is excellent in visualization of the tissue structure, and includes various kinds of images, for example, a proton (hydrogen nucleus) density high-contrast image, a T1 high-contrast image, a T2 high-contrast image, a diffusion high-contrast image, and a vascular image. Meanwhile, the grayscale image (phase image) taking the phase as the pixel value reflects change in the magnetic field resulting from uneven static magnetic field, and difference in the magnetic susceptibility among biotissues. The phase image has been used for adjustment of measurement parameters more often than the use for the diagnosis.
Recently, the use of the phase image that reflects the change in the magnetic field resulting from the magnetic susceptibility difference has been positively applied to the study on estimation of the magnetic susceptibility distribution in vivo from the phase image so that the estimated magnetic susceptibility distribution is used for the image diagnosis. In the brain, for example, the ratio of the paramagnetic substance content differs dependent on the respective tissues. The vein and bleeding part each containing large quantity of deoxyhemoglobin as the paramagnetic substance exhibit higher magnetic susceptibility compared with the surrounding tissue. Therefore, imaging of the magnetic susceptibility distribution in vivo allows imaging of the vein distribution (see Haacke E M, et al., Susceptibility mapping as a means to visualize veins and quantify oxygen saturation, Journal of Magnetic Resonance Imaging vol. 32, pp. 663-676 (2010)), and detection of the microbleed (see Liu T, et al., Cerebral Microbleeds: Burden assessment by using quantitative susceptibility mapping, Radiology, vol. 262, no. 1, pp. 269-278 (2012)).